The invention pertains to the field of printing and in particular to the field of automated loading of printing plates.
In the commercial printing industry, an important step in the preparation of images for printing is the transfer of image information to a substrate that can then be used repeatedly to print the image. While the substrate can take a variety of forms, one of the most extensively used forms is the printing plate, a flat sheet of material with a surface that can be modified in order to selectively retain or repel ink. In general, the modifiable surface is the result of a special coating, commonly referred to as an emulsion, which is a radiation-sensitive coating that changes its properties when exposed to radiation such as visible, ultraviolet, or infrared light. The emulsion coating sits on the surface of a base sheet, which itself may be composed of a variety of materials such as aluminum, polyester, or rubber.
The transfer of image information to a printing plate can be done in a variety of ways. A long-established method is to transfer the image first to a photographic film, and then use the photographic film in order to selectively expose parts of the printing plate to radiation (e.g., visible light), thereby transferring image information to the plate. With the increasing use of information technologies in the graphic arts industry, however, this film-based method is less than efficient for printing images that are stored as computer files. A more recent approach, commonly known as CTP or Computer to Plate, takes advantage of the efficiencies inherent in computerization by transferring the image information directly to the printing plate, eliminating the intermediate step of transferring the image to a photographic film.
The advent of CTP technology is part of an increasing trend towards automation in the printing industry. The increasing use of information technology to create and distribute electronic and print publications, coupled with the more widespread accessibility of such technologies, is contributing to a greater demand for shorter print runs and faster turnaround times. These changes, in turn, have contributed to a greater push toward automating all aspects of the printing process.
Automating the printing industry presents unique technological hurdles. In the case of printing plates, some of these hurdles result from the delicacy of the unexposed emulsion-coated surfaces of these plates. These emulsion-coated surfaces are easily marred, and if marred, create undesirable defects in the final printed product. Attempts to automate the handling of printing plates require measures to prevent damage to the emulsion-coated surfaces of the unexposed plates.
Measures used to reduce marring of plates during storage or transport, however, introduce additional problems for automation. Unexposed plates are typically supplied in packages of 25 to 100 with interleaf sheets, more commonly referred to as xe2x80x9cslip sheetsxe2x80x9d, between the plates. These slip sheets, which may be made of a variety of materials, are used to protect the sensitive printing surfaces of the plates by providing a physical barrier between the emulsion on one plate and the adjacent plate. The slip sheets must be removed from the printing plates prior to imaging.
The need to move sheets of various materials by automated means is not a new problem, and various satisfactory methods for handling sheets have been developed for a number of contexts and industries. The garment industry, for example, uses various combinations of mechanical arid vacuum techniques to pick sheets of textile from the top or bottom of stacks. Office equipment such as photocopiers and printers also employ various means to move individual sheets of paper from a larger stack of paper. Such techniques typically employ friction between rollers and paper surfaces to engage and transport sheets. Within the printing industry, commercial printing presses have long employed similar methods to rapidly feed sheets of paper into sheet-fed printing presses. A long established solution for the feeding of paper into printing presses involves using flexible suction (vacuum) cups in association with other devices to pick up individual paper sheets.
However, the automation of slip sheet removal for the printing industry presents a number of unique problems. In contrast to the examples described above, slip sheet removal is not simply a matter of moving a single sheet from a stack of similar sheets. In general, slip sheets are made of materials different from those used for printing plates, and are further differentiated by being substantially thinner, lighter, and less rigid than the plates they separate. These characteristics also make slip sheets more deformable than their neighboring printing plates. Removing a thin, lightweight, and relatively deformable slip sheet sandwiched between relatively heavy printing plates is a technological challenge further complicated by the fact that the slip sheets must be removed without damaging the sensitive surfaces of the printing plates. The removal process can also be complicated by the fact that slip sheets and plates are often quite large. At present, very large format printing plates can be as large as 58 inches by 80 inches, with correspondingly large sizes for the intervening slip sheets. In addition, the actual materials used for slip sheets can vary, although commonly the slip sheet material is paper, which may be fragile and easily torn.
Another difficulty is that slip sheets tend to adhere to printing plate surfaces when plates are separated from each other. As a result, the exact position of the slip sheet relative to a plate is not consistent. A slip sheet can adhere to the top emulsion-coated surface of a printing plate as it is moved away from its neighboring plate; it may also adhere to the bottom (i.e. non-emulsion-coated) surface of a plate. The tendency of the slip sheets to adhere also complicates slip sheet removal, especially since the slip sheets must be separated from printing plates without scratching or otherwise damaging the emulsion-coated surfaces of the plates. Since the emulsions are very delicate, any mechanical impact imposed upon the emulsion-coated surface of the plate is a potential source of damage, even if it occurs through a slip sheet.
A possible means of addressing this problem is to use suction cups to remove the slip sheets. The use of a vacuum is particularly attractive as it has the potential to eliminate unwanted contact with the sensitive surfaces of the printing plates. A vacuum can be used to draw slip sheets into a desired position from one side without requiring mechanical contact to move the slip sheet. Suction cups have been successfully used in other contexts, for example, to move paper in sheet-fed presses.
However, attempts to remove slip sheets using suction cups tend to fail for a number of reasons:
1. Slip sheets often are so porous that a suction cup cannot achieve sufficient vacuum suction to lift the slip sheet;
2. The slip sheet porosity can lead to a suction cup gripping the non-porous printing plate below, through the slip sheet, and lifting both together;
3. Slip sheets can be very large in some application, such that a slip sheet may be lifted by a suction cup, but the suction cup may not be capable of moving the slip sheet laterally without releasing it;
4. Flexible suction cups have very little peripheral stiffness on their own and rely on the stiffness of the object being picked up to maintain a good seal around the edge of the cup, and to prevent the cup from collapsing on itself. Slip sheets rarely provide sufficient stiffness to permit reliable gripping and are prone to wrinkling at the interface between suction cup and slip sheet, causing vacuum failure and premature release.
There is a need for a method and apparatus for reliably removing slip sheets without causing unnecessary damage to the printing plate surfaces protected by the slip sheets.
In accordance with the present invention, two concentric cylinders, that are mutually rotatable about their common axis, employ suction exerted through two commonly aligned slots in their cylindrical surfaces to separate a deformable sheet from either surface of a flat sheet to which it is adhered. The deformable sheet is then drawn into the recess presented by the two commonly aligned slots. These two concentric cylinders are then rotated su.ch that the deformable sheet is gripped between opposing edges of the two slots. Having gripped the deformable sheet, the removal of the deformable sheet is completed by moving the concentric cylinders over a distance.